The cardiovascular system adapts to exercise as a way of life. The conditioned athlete experiences increases in heart pumping efficiency, blood volume, blood hemoglobin concentration, and the number of mitochondria in muscle fibers. All of these adaptations, improve oxygen delivery to and utilizations by muscle tissue.

An athlete's heart typically changes in response to these increased demands, enlarging as much as 40% or more. Myocardial mass increases, the ventricular cavities expand, and the ventricle walls thicken. Stroke volume increases and heart rate decreases, as does blood pressure. To a physician unfamiliar with a conditioned cardiovascular system, a trained athlete may appear to be abnormal!